The present invention relates to a device for detecting the level of material within an enclosed tank, and, more particularly, to a backscattered radiation system which accurately and automatically determines such level while compensating for variations in the thickness of the walls of the tank.
It is often necessary to measure the level of material within a tank into which entry is impossible. For example, entry may be prohibited, for sanitation reasons, into tanks containing food products. Also, where tanks contain materials under extreme pressure such as liquid propane gas, or at extreme temperature, such as cryogenic liquids, the level of such materials within the tank must be measured from the exterior.
In the past, radiation detection systems have been utilized to measure the level of such materials within enclosed tanks. One such system is a direct radiation system in which a radiation source is placed near the vertical wall of the tank so that it emits gamma radiation into the tank. A radiation detector is placed along the opposite vertical wall of the tank so that radiation from the source passes through one wall of the tank, into the interior of the tank, through the opposite wall of the tank, and is detected by the detector on the opposite side.
Under a direct radiation system, if there is material within the tank at the level at which the source and detector are placed, the amount of radiation sensed by the detector will be less due to radiation absorption by the material. If there is no material in the tank at that level, the amount of radiation detected will be higher, since it will not be absorbed in its travel through the tank. Therefore, the system can be used to determine whether the level of material within the tank is above or below a fixed source and detector. Thus, as the material level in the tank changes, the amount of radiation sensed by the detector will increase or decrease sharply as the upper level of the material passes the source/detector level.
In some direct radiation systems, a single source and detector are moved together along opposite vertical walls of the tank on a pair of pulleys or tracks; however, it is also common for several sources and detectors to be fixed at certain locations along the height of the tank, or to mount elongated or strip sources and detectors in opposing positions along the height of the tank. With all these varieties of direct radiation systems, it is possible to determine the level of material within a tank by noting changes in the amount of radiation detected along the vertical dimension of the tank.
However, the accuracy of all such direct radiation systems is limited by the diameter of the tank. That is, if the system is installed on a very wide tank, the amount of radiation from a usable source may be so low that a reliable measurement is impossible at the opposite wall. The level of radiation may not be increased to overcome this difficulty in many situations, since such increase requires an expensive and very large source assembly.
Another type of level detection system utilizes a backscattered radiation concept in which the radiation source and detector are mounted together on the same side of the tank. In such a system, the source is highly directive and emits radiation only through the wall of the tank and into the interior, where it is backscattered by material within, passes out again through the same wall of the tank, and is detected by a radiation detector mounted adjacent to the source. Thus, if there is material in the tank at the location where the source/detector unit is located, the amount of radiation sensed by the detector is high. However, if there is no material in the tank at that location, the detector will sense a very low amount of radiation since very little is backscattered.
The level of material within the tank can be determined by moving such a backscattered radiation unit up and down along the vertical wall of the tank and noting where an abrupt change in the amount of radiation detected occurs. This backscattered-type detection system solves the problem presented in the direct radiation system; that is, it is not affected by the width of the tank. However, this system does not address a problem which is common to both the direct and the backscattered radiation systems: variations along the vertical dimension of the tank in wall thickness, wall density or other physical properties of the material from which the wall is constructed.
The thickness of the walls of many tanks varies along their height. This is a serious problem in level detection systems which utilize radiation principals, since a thicker tank wall backscatters greater radiation levels and allows less radiation to pass. For example, in a direct radiation system, a very low radiation reading at a certain point may be due to the thickness of the walls of the tank, rather than the presence of material within the tank. An operator of the system may mistake the low radiation reading as indicating that the source/detector pair is at a location below the level of the material, while in actuality, the pair is at a point above the level of the material in the tank.
Similarly, in a backscattered radiation system, a low radiation reading may be due to the thickness of the tank wall rather than the absence of material within the tank. Thus, the system may be at a point at or below the level of material within the tank, but a low radiation reading caused by a thick wall may be mistaken for the absence of material. The adverse effects caused by variations in wall thickness may equally be present due to variations in wall density or other irregularities in the physical properties of the wall.
Therefore, a serious shortcoming of both direct and backscattered radiation level detection systems is that they fail to compensate for wall variations along the height of the tank. This shortcoming greatly effects the accuracy of such systems.
The present invention offers a simple, yet unique solution to this problem by eliminating the adverse effects due to variations in wall thickness or other wall properties of the tank.